Glass microfibers are well known and are discussed in U.S. Pat. No. 4,167,404, which patent is hereby incorporated by reference. It is also known to add glass microfibers to plastics to strengthen and stabilize the plastic while producing parts having very smooth surfaces as disclosed in U.S. patent application Ser. No. 07/758,646 filed Sep. 12, 1991, now Pat. No. 5,232,771, which disclosure is hereby incorporated by reference.
Glass microfibers, because of their various lengths and very small diameters of less than about 4.0 microns, are removed from the air stream in which they are made in bulk form and baled under compression. When removed from the bale the bulk expands to less than about 4 pounds per cubic foot (PCF).
The difficulty of using microfibers in engineered plastics or composites is reducing this bulky mass to a form that will allow the fibers to be metered into the liquid plastic at a commercially feasible rate and cost while maintaining the characteristics of the fibers that provide stability, rigidity and strength to the plastic composite. Others have found that the microfiber bulk fiber could be reduced to a form suitable for laboratory work using a high speed tow cutter or chopper similar to the device shown in U.S. Pat. No. 3,815,461. This process produced a fiber product that could be introduced into plastic and the reinforced plastic parts made from this product had the properties desired.
The fiber product produced by this chopper had a bulk density of about 8.1 PCF, but this nodulating method required hand feeding of the bulk fiber to the chopper and the chopping process was very dusty since this equipment is not very amendable to an enclosed system. Another big problem was that the blades became too dull to cut after processing only about 50 pounds requiring shut down to change blade rolls. This situation was too costly and a much better attrition method was needed to make the product commercially feasible.
It is known to ball mill or hammer mill large diameter glass fibers, such as 10-23 microns, into very short fibers to put into plastics. However, when glass microfibers are so processed the properties are not retained or the fiber is not sufficiently agglomerated to a form that can be accurately metered and rapidly fed into the extruder of a plastic pellet making machine. Ball milling was investigated using an 1-HSA Attritor made by the Union Process Company of Akron, Ohio. The mill was successful in overcoming most of the process problems encountered with the chopper, but the properties in the plastic parts were unsatisfactorily degraded. The best properties were achieved using a milled microfiber having a bulk density of about 3.7 PCF, which is lower than desired and the plastic properties still were not satisfactory.
Hammer milling produced a product resembling goose down which was too low in density and too fluffy to facilitate metering and feeding to an extruder, particularly at practical rates.